The use of an intense beam of radiation to impose predetermined patterns or materials on target specimens requires a system that coordinates the emission and position of the beam. Lasers, electron guns, and high energy ion streams are examples of beam sources that require coordination between beam emission and position.
Conventional radiation beam position and emission coordination systems typically use a servomechanism, a simple logic function network, and an incremental position encoder to coordinate beam emission and position. Beam pulses are timed by a variable frequency clock while a separate, fixed frequency clock times the servomechanism movement between successive positions.
To move a laser beam, for example, along a predetermined path, data comprising the intended beam travel pattern are first resolved into a series of pattern data segments ("data segments"). Each data segment includes servomechanism trajectory information sufficient to command the servo through an intended beam motion along a corresponding target segment. Additionally, the data segment contains beam impact information used to direct whether and where the beam should impact the target.
As the servo moves the laser or target in correspondence with each data segment, and sequentially from data segment to data segment, the incremental encoder resolves measured beam position into beam position coordinate data. Whenever the beam position coordinate data coincide with a predetermined laser beam impact site, the servo provides to the laser excitation circuit a signal that starts pulsed laser beam emission. Whenever measured position matches a stored laser beam stop site, the servo provides to the laser excitation circuit a signal that ceases laser beam emission.
The data segments are devised so that the laser beam is either "on" or "off" throughout the entire segment. Therefore, laser beam emission should take place at the beginning and the end of each servo movement through a corresponding target segment. The beginning and end of a servo movement are timed by the fixed frequency clock. Therefore, if initial laser beam impact is intended at the beginning of a target segment move, a pulse request signal is sent to the excitation circuit when servo movement begins. Servo movement is timed from the fixed frequency clock; however, the occurrence of pulsed beam emission is derived from the variable frequency clock. The pulse request signal produced by the servo may occur when the separate variable frequency clock is between pulses. Consequently, beam emission may not start or stop until the variable frequency clock changes state.
There would, therefore, be a delay between the start or stop of the servo movement and pulsed laser beam emission. Consequently, the initial laser beam pulse may strike the substrate at a point after the beginning of a target segment or the last laser beam pulse may occur at a point before the end of the segment.
The asynchronous relationship between positional control and pulse emission becomes acute when two segments requiring laser beam emission are expected to meet. In such cases, there will be no laser beam impact at the intended junction between the two segments, thereby resulting in a gap between them.
Such gaps are particularly significant during the fabrication of electronic and optical devices requiring submicron precision. Integrated circuits, for example, are often fabricated to include multiple iterations of redundant circuit elements, such as memory cells. Such circuits are tested for cell-to-cell integrity. When defective cells are discovered, their locations are mapped by a program whose output can be used to control a laser beam to cut appropriate circuit links and thereby excise the defective cells and substitute replacement redundant cells. Because integrated circuit elements are frequently fabricated in submicron dimensions, such laser cutting operations require close control of beam impact position. Consequently, temporal or spatial inaccuracy in starting or stopping a train of laser beam pulses can lead to failure to excise, or implement, selected memory cells in the integrated circuit.
Additionally, integrated electronic circuits are fabricated with interconnected junctions of differentially doped semiconductor areas. Such junctions may be fabricated using either a diffusion process to grow ionic layers or high energy ion beams to implant ions directly into the substrate.
However, diffusion techniques grow ions vertically and laterally; such lateral diffusion blurs intended feature perimeters. In contrast, ion implantation exhibits no lateral diffusion; therefore, ion beam techniques are frequently preferred for small dimension junction formation. Consequently, techniques that improve the dimensional control of ion beam targeting lead to improved fabrication of small junction regions. As a particular example, emitter regions of transistors are sometimes smaller than other transistor features. Therefore, in very large scale integration techniques where small emitters are needed, temporal and spatial synchronization between ion beam excitation and position is critical.
Another solution to correlate beam emission and position uses an independent logic network to monitor a single axis at high sample rates. An incremental encoder resolves beam position into determined position coordinates. As the servo moves the beam across the target substrate according to encoder coordinate data, the independent logic circuit simultaneously compares position encoder data to predetermined beam pulse start and stop locations stored in random access memory. When a determined position corresponds to a beam start location, the logic network produces a signal that starts beam emission.
Such systems have inherent limitations. Since determined position data are compared to stored location data before the network starts beam emission, there is slew error as the servo moves the laser before emission can start. Compensation may be made for significant components of such errors; however, additional errors arise. Monitor board logic and servo logic independently decode the encoder data; therefore, a decoding timing differential is induced that is unpredictable and difficult to correct. Moreover, each added logic network can monitor only a single axis.
Both such prior art systems are also limited by the difference between position as determined from encoder coordinate data and predetermined location as stored in memory. Digital encodation of a dimensional continuum across a target will necessarily result in errors unless interpolation techniques are used, which result in additional financial and speed expenses.